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Author Topic: HSC Chemistry Question Thread  (Read 1040632 times)  Share 

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anotherworld2b

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Re: Chemistry Question Thread
« Reply #450 on: July 22, 2016, 01:24:13 am »
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How would you do q5c and d?
I got the answers for a and b but i didnt know how to do c and d.

Help is greatly appreciated

RuiAce

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Re: Chemistry Question Thread
« Reply #451 on: July 22, 2016, 01:32:22 am »
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How would you do q5c and d?
I got the answers for a and b but i didnt know how to do c and d.

Help is greatly appreciated
Consider the original balanced equations to know what is going on first.

c) This is a metal and acid reaction. We know that:
Metal + Acid -> Salt + Hydrogen gas

So treating our species appropriately:

Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)

Have any charges changed?
Zn: It has went from zinc solid to the zinc ion
SO4: The sulfate ion is as is
H: The hydrogen ion has become hydrogen gas.

So we have Zn(s) + 2 H+ -> Zn2+ + H2(g)

c) This is a precipitation reactants. But it's made easier because they give us the reactants and the product.
Pb(NO3)2 + 2 KNO3(aq) + PbI2(s)

Have any changed occurred?
Pb: It stayed as Pb2+ but it precipitated into the product PbI2(s) which cannot be split!
NO3: The nitrate ion is as is
K: The potassium ion is as is
I: It stayed as I- but it precipitated into the product PbI2(s) which cannot be split!

So we have Pb2+ + 2 I- -> PbI2(s)
_____________

An alternate way to approach these questions is to write out the complete ionic equation based off the balanced equation and then cancel out any spectator ions.

For c) this would be

Zn(s) + 2 H+ + SO42- -> Zn2+ + SO42- + H2(g)

conic curve

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Re: Chemistry Question Thread
« Reply #452 on: July 22, 2016, 01:16:14 pm »
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I need help with the water topic since I find it quite difficult

Q. A soluble molecular substance such as sugar (e.g. sucrose C11H22O11) is mixed with water

a. Identify the forces that mist break for it to dissolve

I was not sure of this and was thinking about intermolecular or intramolecular but I seriously don't know.

b. Describe the attraction that then develops between the free water and sugar molecules

I'm not sure of this. Is it Hydrogen bonding?

Thanks guys  ;)

RuiAce

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Re: Chemistry Question Thread
« Reply #453 on: July 22, 2016, 01:25:52 pm »
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I need help with the water topic since I find it quite difficult

Q. A soluble molecular substance such as sugar (e.g. sucrose C11H22O11) is mixed with water

a. Identify the forces that mist break for it to dissolve

I was not sure of this and was thinking about intermolecular or intramolecular but I seriously don't know.

b. Describe the attraction that then develops between the free water and sugar molecules

I'm not sure of this. Is it Hydrogen bonding?

Thanks guys  ;)
Draw out the molecular structure of sucrose. It is full of polar OH bonds everywhere. The fact that they are polar means that sucrose molecules will engage in dipole-dipole interactions (and maybe a small amount of hydrogen bonding) with each other. (Do you see why?)

If water is to dissolve sucrose, the intermolecular forces between sucrose molecules must be broken as water has to start interacting intermolecularly with sucrose instead. (No intramolecular bonds should be broken or we no longer have sucrose).

Because the polarity of water is much stronger than that of sucrose, it will overcome the intermolecular forces between sucrose molecules and form extensive hydrogen bonding with sucrose molecules instead, thereby dissolving the substance.


Process of thought:
a) Water has to break something. What is the only logical thing to break? In fact, what even is there to break is a better question
b) It is hydrogen bonding. Just draw out the shape of sucrose and water and you should be able to realise how the polar -OH groups on sucrose can interact with water.

amandali

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Re: Chemistry Question Thread
« Reply #454 on: July 22, 2016, 01:48:53 pm »
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dont know how to do q17 b

RuiAce

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Re: Chemistry Question Thread
« Reply #455 on: July 22, 2016, 02:00:42 pm »
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(Image removed from quote.)

dont know how to do q17 b
Please give the answers to a next time if you could do it. Would make life easier :) (especially since I might've gotten it wrong).


Because the equilibrium constant is a decently large number, at equilibrium there should be more of the products than the reactants.

Firstly, our calculations implicitly verify this as in the event the concentration of oxygen is greater, there would indeed be more of SO2 being converted into SO3 (90% instead of 50%).

How has Le Chatelier's principle been obeyed here? I reckon it's actually quite simple. We are saying that if there's more O2 (the reactant), then the forward reaction should be favoured. The equilibrium shifts towards the right, converting SO2 to SO3 in the process. If the concentration of O2 is greater, indeed we should anticipate that the forward reaction will occur to a greater extent.

marynguyen18

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Re: Chemistry Question Thread
« Reply #456 on: July 22, 2016, 09:25:58 pm »
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Hey i was just wondering how would you structure a 4-7 mark question on Cellulose?

conic curve

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Re: Chemistry Question Thread
« Reply #457 on: July 22, 2016, 09:27:07 pm »
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Hey i was just wondering how would you structure a 4-7 mark question on Cellulose?

It really depends on what the question is and what the marking criteria is

marynguyen18

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Re: Chemistry Question Thread
« Reply #458 on: July 22, 2016, 09:34:08 pm »
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What if the question on Cellulose is related to it as being an alternative to petrochemicals and its potential as a raw material for chemicals used in industry?

RuiAce

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Re: Chemistry Question Thread
« Reply #459 on: July 22, 2016, 09:41:57 pm »
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What if the question on Cellulose is related to it as being an alternative to petrochemicals and its potential as a raw material for chemicals used in industry?
Think about why it can be an alternative to petrochemicals. This goes back down to the fact that natural production of ethylene comes from fossil fuels = bad for reasons you should already know.

Cellulose is a renewable resource. If you want to produce ethanol, all you have to do is ferment it and done. And you can easily get more cellulose from dead plant matter.

(But you should always analyse the negatives of this process, e.g. lots of arable land needed and the energy required to trigger fermentation is massive.)

Essentially, the part about being an alternative to petrochemicals is a process of outweighing the advantages and disadvantages against burning fossil fuels. The raw materials part can be related to things such as

- Ethanol: Very powerful solvent used in a variety of scenarios
- Polyethylene: A common polymer that we use today for plastics

Also, make sure to give a judgement if it ever asks you to evaluate/assess.

marynguyen18

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Re: Chemistry Question Thread
« Reply #460 on: July 22, 2016, 09:46:18 pm »
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Thank you so much! Also when you make a judgement does it mean that you have to say what you think would be a good alternative?

RuiAce

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Re: Chemistry Question Thread
« Reply #461 on: July 22, 2016, 09:49:01 pm »
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You make a judgment based off the question.

E.g. if the question is about the potentials of cellulose in being a replacement, you would recollect the info you have and say hence it is most likely that it is a good/bad alternative. Or something like overall, it is (say, adequate) in that it is able to...

Your judgment doesn't have to be "correct" - it just has to make sense and link to the question.


conic curve

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Re: Chemistry Question Thread
« Reply #462 on: July 22, 2016, 09:50:30 pm »
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Thank you so much! Also when you make a judgement does it mean that you have to say what you think would be a good alternative?

Judgement really means to give your own opinion

Anyways can anyone give me tips on how to answer hydrocarbon questions because I really suck at that

Thanks
« Last Edit: July 22, 2016, 09:54:42 pm by conic curve »

RuiAce

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Re: Chemistry Question Thread
« Reply #463 on: July 22, 2016, 09:59:18 pm »
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Judgement really means to give your own opinion

Anyways can anyone give me tips on how to answer hydrocarbon questions because I really suck at that

Thanks
Before commencing, your prefixes of meth-, eth-, prop-, but-, pent- etc. should be known off by heart first.

The alkane series only has single bonds. The key characteristic of the alkenes series is that somewhere in there, there is a double bond.

A complete structural formula includes the bonds. 1-pentene is basically saying that there is a double bond, coming off carbon #1.
The correct answer is

C=C-C-C-C

with H's coming off the carbons. (Note: if you have a double bond, do not draw in too many H's)

A condensed formula just writes out what's there. For 1-pentene it would be
CH2CHCH2CH2CH3

For heptane this would be
CH3CH2CH2CH2CH2CH2CH3 (count the carbons! The prefix matters!)

For 1-heptene this would be
CH3CH2CHCHCH2CH2CH3

This one might not be obvious. It helps to draw out the complete structural formula whenever you're stuck.


Molecular formulae are easy. Alkanes have formula CnH2n+2, whereas alkenes have formula C2H2n

I'm leaving stuff unanswered because I want you to do some work yourself.

conic curve

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Re: Chemistry Question Thread
« Reply #464 on: July 22, 2016, 10:03:37 pm »
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Before commencing, your prefixes of meth-, eth-, prop-, but-, pent- etc. should be known off by heart first.

The alkane series only has single bonds. The key characteristic of the alkenes series is that somewhere in there, there is a double bond.

A complete structural formula includes the bonds. 1-pentene is basically saying that there is a double bond, coming off carbon #1.
The correct answer is

C=C-C-C-C

with H's coming off the carbons. (Note: if you have a double bond, do not draw in too many H's)

A condensed formula just writes out what's there. For 1-pentene it would be
CH2CHCH2CH2CH3

For heptane this would be
CH3CH2CH2CH2CH2CH2CH3 (count the carbons! The prefix matters!)

For 1-heptene this would be
CH3CH2CHCHCH2CH2CH3

This one might not be obvious. It helps to draw out the complete structural formula whenever you're stuck.


Molecular formulae are easy. Alkanes have formula CnH2n+2, whereas alkenes have formula C2H2n

I'm leaving stuff unanswered because I want you to do some work yourself.

Yes I know that but when it comes to answering them, I get stuck

Could you at least give me hints on them?

What does it mean by like 3-methy-2-difluro-1-ene (or something similar to that)?
« Last Edit: July 22, 2016, 10:07:01 pm by conic curve »